In the mining industry, the ore concentrate produced at the mine must generally be transported or shipped to other premises for final refining by either electrolysis, smelting etc. The costs associated with transporting and handling these concentrates are not negligible, and attempts have been made to reduce them to the minimum. One specific issue is the control of the moisture content. Such control is critical because it has direct consequences on handling and transportation of the concentrates. Concentrates too dry generate undesirable dust, while excessive moisture content causes the concentrates to stick to the container surfaces, and significantly increases shipping costs because of greater weight. In the mining industry, a generally accepted moisture content for ore concentrates is about 7-9%. Drying of ore concentrates is sometimes carried out in an uncontrolled manner, resulting in a moisture content varying from 5 to 12%. Overdrying concentrates significantly increases energy costs.
Several physical principles and properties have been applied to measure moisture content of various particulate materials. For example, measurements based on electrical conductivity, dielectric properties, microwave absorption etc. are known. They are all affected by the electrical properties of both the concentrate and water, and therefore cannot be employed reliably for conductive concentrates such as copper and lead concentrates. Errors in conductivity-based measurement depend on both dissolved impurities in the water and variations in the nominal concentrate composition.
Sensors using infra-red based measurements have also been developed, but they are affected by the concentrate's optical absorption at wavelengths used to characterize the water content. Further, since measurements take place at the sample's surface, surface drying may prevent measurements from accurately representing the bulk. Finally, water vapor between the sample and the sensor may also adversely influence accuracy.
The sensor disclosed in U.S. Pat. No. 3,800,141 takes measurements based on the selective deceleration of fast neutrons in conjunction with absorption of gamma radiation to establish density. Such measurement technique is complex, potentially hazardous due to the use of nuclear sources, and, as for all the sensors described above, tends to be sensitive to sample presentation.
An excellent review of moisture sensors is provided in Chapter 5 of Moisture sensors in process control, authored by K. Carr-Brion (Elsevier Applied Science Publishers). It is mentioned therein that mechanical properties of powders can be used to determine the moisture content therein. For example, the determination of the moisture content in sand is exemplified by correlating the water content with the capacity of sand to pass across slots of different widths. Although methods and sensors using mechanical properties are simple, reliable and relatively cheap, they are, as pointed out by Carr-Brion, highly specific, and require complicated engineering for coupling with computerized control units.
It would therefore be highly desirable to develop a sensor not affected by the electrical and/or optical properties of either the particulate material, or associated moisture. Such sensor would preferably use mechanical properties or drag forces and would be used in a continuous manner. Because the correlation between dust and sticking properties and moisture is incidental, such novel sensor would be particularly useful where moisture control is intended to improve material handling with respect to dust and sticking. Finally, to avoid any misreading or misinterpretation, the sensor should be placed or buried within the particulate material, so that the measurements are representative of the moisture content of the bulk of the particulate material.